Magnetic stack having reference layers with orthogonal magnetization orientation directions

ABSTRACT

A magnetic cell includes a ferromagnetic free layer having a free magnetization orientation direction and a first ferromagnetic pinned reference layer having a first reference magnetization orientation direction that is parallel or anti-parallel to the free magnetization orientation direction. A first oxide barrier layer is between the ferromagnetic free layer and the first ferromagnetic pinned reference layer. The magnetic cell further includes a second ferromagnetic pinned reference layer having a second reference magnetization orientation direction that is orthogonal to the first reference magnetization orientation direction. The ferromagnetic free layer is between the first ferromagnetic pinned reference layer and the second ferromagnetic pinned reference layer.

CROSS-REFERENCE

This application is a continuation of application Ser. No. 12/502,209,filed Jul. 13, 2009, the contents of each is hereby incorporated byreference in its entirety.

BACKGROUND

Spin torque transfer technology, also referred to as spin electronics,combines semiconductor technology and magnetics, and is a more recentdevelopment. In spin electronics, the spin of an electron, rather thanthe charge, is used to indicate the presence of digital information. Thedigital information or data, represented as a “0” or “1”, is storable inthe alignment of magnetic moments within a magnetic element. Theresistance of the magnetic element depends on the moment's alignment ororientation. The stored state is read from the element by detecting thecomponent's resistive state.

The magnetic element, in general, includes a ferromagnetic pinned layerand a ferromagnetic free layer, each having a magnetization orientationthat defines the resistance of the overall magnetic element. Such anelement is generally referred to as a “spin tunneling junction,”“magnetic tunnel junction”, “magnetic tunnel junction cell”, and thelike. When the magnetization orientations of the free layer and pinnedlayer are parallel, the resistance of the element is low. When themagnetization orientations of the free layer and the pinned layer areantiparallel, the resistance of the element is high.

Application of spin torque transfer memory has a switching currentdensity requirement generally at 10⁶ to 10⁷ A/cm², which leads todifficulty in integrating with a regular CMOS process. It is desirableto reduce the switching current density significantly in order to make afeasible product. Various attempts have been made.

However, there is a dilemma between switching current and data stabilityin spin torque transfer cells. A low switching current can reduce dataretention due to thermal instability of the spin torque transfer cells.Spin torque transfer cell design that can achieve both low switchingcurrent with sufficient data retention is desired.

BRIEF SUMMARY

The present disclosure relates to magnetic cells, such as a spin torquememory cell, that have magnetic two reference layers or elements thathave orthogonal magnetization orientation directions. These spin torquememory cells quickly switch between a high resistance data state and alow resistance data state and include a free magnetic layer between twooxide barrier layers. The two reference layers are alignedperpendicularly.

In an embodiment of this disclosure is a magnetic cell that includes aferromagnetic free layer having a free magnetization orientationdirection and a first ferromagnetic pinned reference layer having afirst reference magnetization orientation direction that is parallel oranti-parallel to the free magnetization orientation direction. A firstoxide barrier layer is between the ferromagnetic free layer and thefirst ferromagnetic pinned reference layer. The magnetic cell furtherincludes a second ferromagnetic pinned reference layer having a secondreference magnetization orientation direction that is orthogonal to thefirst reference magnetization orientation direction. The ferromagneticfree layer is between the first ferromagnetic pinned reference layer andthe second ferromagnetic pinned reference layer.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1A is a schematic side view diagram of a magnetic cell in a lowresistance data state and with orthogonal reference layer magnetizationorientations;

FIG. 1B is a schematic side view diagram of a magnetic cell in a highresistance data state and with orthogonal reference layer magnetizationorientations;

FIG. 2 is a schematic diagram of an illustrative memory unit including amemory cell and a semiconductor transistor;

FIG. 3 is a schematic diagram of an illustrative memory array;

FIG. 4 is a schematic side view diagram of another magnetic cell withorthogonal reference layer magnetization orientations; and

FIG. 5 is a schematic side view diagram of another magnetic cell withorthogonal reference layer magnetization orientations.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

This disclosure is directed to magnetic stacks or cells (e.g., spintorque memory (STRAM) cells) having magnetic two reference layers orelements that have orthogonal magnetization orientation directions.These spin torque memory cells quickly switch between a high resistancedata state and a low resistance data state and include a free magneticlayer between two oxide barrier layers. The two reference layers arealigned perpendicularly. This data cell construction increases the writespeed and improves the tunneling magneto-resistance ratio of the datacell over conventional data cells that do not have perpendicularlyaligned reference layers.

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.Any definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

It is noted that terms such as “top”, “bottom”, “above, “below”, etc.may be used in this disclosure. These terms should not be construed aslimiting the position or orientation of a structure, but should be usedas providing spatial relationship between the structures.

While the present disclosure is not so limited, an appreciation ofvarious aspects of the disclosure will be gained through a discussion ofthe examples provided below.

FIG. 1A is a schematic side view diagram of a magnetic cell 10 in a lowresistance data state and with orthogonal reference layer magnetizationorientations. FIG. 1B is a schematic side view diagram of a magneticcell 10 in a high resistance data state and with orthogonal referencelayer magnetization orientations. The magnetic tunnel junction cell 10includes a first ferromagnetic pinned reference layer or element 14having a first reference magnetization orientation direction M_(R1), aferromagnetic free element or layer 18 having a free magnetizationorientation direction M_(F) and a first tunneling barrier 16 separatingthe first ferromagnetic pinned reference magnetic element 14 from theferromagnetic free element 18. A second ferromagnetic pinned referencelayer or element 13 has a second reference magnetization orientationdirection M_(R2) that is orthogonal to the first reference magnetizationorientation direction M_(R1). The ferromagnetic free layer is betweenthe first ferromagnetic pinned reference layer 14 and the secondferromagnetic pinned reference layer 13. In many embodiments, a secondtunneling barrier 15 separates the second ferromagnetic pinned referencemagnetic element 13 from the ferromagnetic free element 18.

These elements or layers are disposed electrically between a firstelectrode 13 and a second electrode 19. While a single magnetic tunneljunction cell 10 is shown, it is understood that a plurality of magnetictunnel junction cell 10 can be arranged in an array to form a memoryarray. Other layers, such as seed or capping layers, are not depictedfor clarity.

The ferromagnetic free element 18 has a free magnetization orientationdirection M_(F) that is switchable between a high resistance data state(i.e., anti-parallel direction relative to the first ferromagneticpinned reference magnetic element 14 magnetization orientation directionM_(R1) and illustrated in FIG. 1B) and a low resistance data state(i.e., parallel direction relative to the first ferromagnetic pinnedreference magnetic element 14 magnetization orientation direction M_(R1)and illustrated in FIG. 1A). The ferromagnetic free element or layer 18,first ferromagnetic pinned reference magnetic element 14, and secondferromagnetic pinned reference magnetic element 13 have in-planemagnetic anisotropy.

While the first ferromagnetic pinned reference element 14 is illustratedas a single layer, it is understood that this element 14 can include twoor more layer such as, a ferromagnetic reference (pinned) layer and aantiferromagnetic reference (pinning) layer, where the antiferromagneticreference layer serves to fix the magnetization of the ferromagneticreference layer. In other embodiments, the first ferromagnetic pinnedreference element 14 includes more than one ferromagnetic layer that arecoupled anti-ferromagnetically to each other (e.g., syntheticantiferromagnet). The ferromagnetic reference layer can be formed of anyuseful material such as, for example, alloys and materials including Co,Fe, and/or Ni. Ternary alloys, such as CoFeB, may be particularly usefulbecause of their lower moment and high polarization ratio, which aredesirable for the spin-current switching. The antiferromagneticreference layer can be formed of any useful material such as, forexample, IrMn, FeMn, and/or PtMn.

While the second ferromagnetic pinned reference element 13 isillustrated as a single layer, it is understood that this element 13 caninclude two or more layer such as, a ferromagnetic reference (pinned)layer and an antiferromagnetic reference (pinning) layer, where theantiferromagnetic reference layer serves to fix the magnetization of theferromagnetic reference layer. In other embodiments, the secondferromagnetic pinned reference element 13 includes more than oneferromagnetic layer that are coupled anti-ferromagnetically to eachother (e.g., synthetic antiferromagnet). The ferromagnetic referencelayer can be formed of any useful material such as, for example, alloysand materials including Co, Fe, and/or Ni. Ternary alloys, such asCoFeB, may be particularly useful because of their lower moment and highpolarization ratio, which are desirable for the spin-current switching.The antiferromagnetic reference layer can be formed of any usefulmaterial such as, for example, IrMn, FeMn, and/or PtMn.

The ferromagnetic free element 18 can be formed of any useful softmagnetic material that allows a magnetization orientation of theferromagnetic free element 18 to switch between a first magnetizationorientation and an opposing second magnetization orientation. In manyembodiments the ferromagnetic free element 18 is formed of a CoFeBmaterial such as, Co₆₅Fe₃₀B₁₅ and having a magnetic saturation in arange from 1200 to 500 emu/cc, for example. The first magnetizationorientation can be parallel with a magnetization orientation of thefirst ferromagnetic pinned reference element 14, forming a lowresistance data state or a “0” data state. The second magnetizationorientation can be anti-parallel with a magnetization orientation of thefirst ferromagnetic pinned reference element 14, forming a highresistance data state or a “1” data state. The ferromagnetic free layercan be formed of any useful material such as, for example, alloys andmaterials including Co, Fe, and/or Ni. Ternary alloys, such as CoFeB,may be particularly useful because of their lower moment and highpolarization ratio, which are desirable for the spin-current switching.Thus the ferromagnetic free element 18 can be switched due to spintorque transfer induced by a current passing through the magnetic cell10.

The first and second tunneling or oxide barrier 15, 16 is anelectrically insulating and non-magnetic material. The tunneling oroxide barrier 15, 16 can be formed of any useful electrically insulatingand non-magnetic material such as, AlO, MgO, and/or TiO, for example. Insome embodiments, the oxide barrier layers 15, 16 have a thickness ofabout 0.5-2 nm.

Electrodes 13, 19 electrically connect the magnetic tunnel junction cell10 to a control circuit providing read and write currents through themagnetic tunnel junction cell 10. Resistance across the magnetic tunneljunction cell 10 is determined by the relative orientation of themagnetization vectors or magnetization orientations of ferromagneticlayers 14, 18. The magnetization directions of the ferromagnetic pinnedreference layers 14, 13 are pinned in a predetermined direction whilethe magnetization direction of ferromagnetic free layer 18 is free torotate under the influence of spin torque when a current flows throughthe magnetic tunnel junction cell 10.

Switching the resistance state and hence the data state of magnetictunnel junction cell 10 via spin-torque transfer occurs when a current,passing through a magnetic layer of magnetic tunnel junction cell 10,becomes spin polarized and imparts a spin torque on the ferromagneticfree layer 18 of magnetic tunnel junction cell 10. When a sufficientspin torque is applied (sufficient to overcome the energy barrier E) toferromagnetic free layer 18, the magnetization orientation of theferromagnetic free layer 18 can be switched between two oppositedirections and accordingly, magnetic tunnel junction cell 10 can beswitched between the parallel state (i.e., low resistance state or “0”data state) and anti-parallel state (i.e., high resistance state or “1”data state).

FIG. 2 is a schematic diagram of an illustrative memory unit including amemory unit 20 and a semiconductor transistor 22. Memory unit 20includes a magnetic tunnel junction cell 10, as described herein,electrically coupled to semiconductor transistor 22 via an electricallyconducting element 24. Transistor 22 includes a semiconductor substrate21 having doped regions (e.g., illustrated as n-doped regions) and achannel region (e.g., illustrated as a p-doped channel region) betweenthe doped regions. Transistor 22 includes a gate 26 that is electricallycoupled to a word line WL to allow selection and current to flow from abit line BL to memory cell 10. An array of memory units 20 can be formedon a semiconductor substrate utilizing semiconductor fabricationtechniques.

FIG. 3 is a schematic diagram of an illustrative memory array 30. Memoryarray 30 includes a plurality of word lines WL and a plurality of bitlines BL forming a cross-point array. At each cross-point a memory cell10, as described herein, is electrically coupled to word line WL and bitline BL. A select device (not shown) can be at each cross-point or ateach word line WL and bit line BL.

FIG. 4 is a schematic side view diagram of another magnetic cell 40 withorthogonal reference layer magnetization orientations. The magnetictunnel junction cell 40 includes a first ferromagnetic pinned referencelayer or element 14 having a first reference magnetization orientationdirection, a ferromagnetic free element or layer 18 having a freemagnetization orientation direction and a first tunneling barrier 16separating the first ferromagnetic pinned reference magnetic element 14from the ferromagnetic free element 18. A second ferromagnetic pinnedreference layer or element 13 has a second reference magnetizationorientation direction that is orthogonal to the first referencemagnetization orientation direction. The ferromagnetic free layer 18 isbetween the first ferromagnetic pinned reference layer 14 and the secondferromagnetic pinned reference layer 13. In many embodiments, a secondtunneling barrier 15 separates the second ferromagnetic pinned referencemagnetic element 13 from the ferromagnetic free element 18.

These elements or layers are disposed electrically between a firstelectrode 13 and a second electrode 19. While a single magnetic tunneljunction cell 10 is shown, it is understood that a plurality of magnetictunnel junction cell 10 can be arranged in an array to form a memoryarray. Other layers, such as seed or capping layers, are not depictedfor clarity.

The first ferromagnetic pinned reference layer or element 14 includes afirst synthetic anti-ferromagnetic element SAF1 and a firstantiferromagnetic reference (pinning) layer AFM1. The first syntheticanti-ferromagnetic element SAF1 includes two ferromagnetic layers FM1,FM2 anti-ferromagnetically coupled and separated by a non-magnetic andelectrically conducting spacer layer SP1. The second ferromagneticpinned reference layer or element 13 includes a second syntheticanti-ferromagnetic element SAF2 and a second antiferromagnetic reference(pinning) layer AFM2. The second synthetic anti-ferromagnetic elementSAF2 includes two ferromagnetic layers FM3, FM4 anti-ferromagneticallycoupled and separated by a non-magnetic and electrically conductingspacer layer SP2.

In many embodiments the first antiferromagnetic reference (pinning)layer AFM1 has a different material composition than the secondantiferromagnetic reference (pinning) layer AFM2. The firstantiferromagnetic reference (pinning) layer AFM1 can have a greaterblocking temperature than the second antiferromagnetic reference(pinning) layer AFM2. Thus the first ferromagnetic pinned referencelayer or element 14 can have its magnetization orientation set at ahigher temperature than the later formed second ferromagnetic pinnedreference layer or element 13. Then the second ferromagnetic pinnedreference layer or element 13 can have its magnetization orientation setat a lower temperature than the prior formed first ferromagnetic pinnedreference layer or element 14.

FIG. 5 is a schematic side view diagram of another magnetic cell 50 withorthogonal reference layer magnetization orientations. The magnetictunnel junction cell 50 includes a first ferromagnetic pinned referencelayer or element 14 having a first reference magnetization orientationdirection, a ferromagnetic free element or layer 18 having a freemagnetization orientation direction and a first tunneling barrier 16separating the first ferromagnetic pinned reference magnetic element 14from the ferromagnetic free element 18. A second ferromagnetic pinnedreference layer or element 13 has a second reference magnetizationorientation direction that is orthogonal to the first referencemagnetization orientation direction. The ferromagnetic free layer 18 isbetween the first ferromagnetic pinned reference layer 14 and the secondferromagnetic pinned reference layer 13. In many embodiments, a secondtunneling barrier 15 separates the second ferromagnetic pinned referencemagnetic element 13 from the ferromagnetic free element 18.

These elements or layers are disposed electrically between a firstelectrode 13 and a second electrode 19. While a single magnetic tunneljunction cell 10 is shown, it is understood that a plurality of magnetictunnel junction cell 10 can be arranged in an array to form a memoryarray. Other layers, such as seed or capping layers, are not depictedfor clarity.

The first ferromagnetic pinned reference layer or element 14 includes afirst synthetic anti-ferromagnetic element SAF1 and a antiferromagneticreference (pinning) layer AFM. The first synthetic anti-ferromagneticelement SAF1 includes two ferromagnetic layers FM1, FM2anti-ferromagnetically coupled and separated by a non-magnetic andelectrically conducting spacer layer SP1. The second ferromagneticpinned reference layer or element 13 includes a second syntheticanti-ferromagnetic element SAF2 and a permanent magnet PM. The secondsynthetic anti-ferromagnetic element SAF2 includes two ferromagneticlayers FM3, FM4 anti-ferromagnetically coupled and separated by anon-magnetic and electrically conducting spacer layer SP2. Themagnetization orientation of the first ferromagnetic pinned referencelayer or element 14 can be set with a magnetic set anneal and themagnetization orientation of the second ferromagnetic pinned referencelayer or element 13 can be set with the permanent magnet PM.

The various structures of this disclosure may be made by thin filmtechniques such as chemical vapor deposition (CVD), physical vapordeposition (PVD), sputter deposition, and atomic layer deposition (ALD).

Thus, embodiments of the MAGNETIC STACK HAVING REFERENCE LAYERS WITHORTHOGONAL MAGNETIZATION ORIENTATION DIRECTIONS are disclosed. Theimplementations described above and other implementations are within thescope of the following claims. One skilled in the art will appreciatethat the present disclosure can be practiced with embodiments other thanthose disclosed. The disclosed embodiments are presented for purposes ofillustration and not limitation, and the present invention is limitedonly by the claims that follow.

The use of numerical identifiers, such as “first”, “second”, etc. in theclaims that follow is for purposes of identification and providingantecedent basis. Unless content clearly dictates otherwise, it shouldnot be implied that a numerical identifier refers to the number of suchelements required to be present in a device, system or apparatus. Forexample, if a device includes a first layer, it should not be impliedthat a second layer is required in that device.

1. A magnetic cell comprising: a ferromagnetic free layer having a freemagnetization orientation direction; a first ferromagnetic pinnedreference layer having a first reference magnetization orientationdirection that is parallel or anti-parallel to the free magnetizationorientation direction; a first oxide barrier layer between theferromagnetic free layer and the first ferromagnetic pinned referencelayer; and a second ferromagnetic pinned reference layer comprising apermanent magnet and having a second reference magnetization orientationdirection that is orthogonal to the first reference magnetizationorientation direction, the ferromagnetic free layer between the firstferromagnetic pinned reference layer and the second ferromagnetic pinnedreference layer.
 2. The magnetic cell of claim 1 wherein theferromagnetic free layer, first ferromagnetic pinned reference layer,and second ferromagnetic pinned reference layer, have in-plane magneticanisotropy.
 3. The magnetic cell of claim 1 further comprising a secondoxide barrier layer between the ferromagnetic free layer and the secondferromagnetic pinned reference layer.
 4. The magnetic cell of claim 1wherein the first ferromagnetic pinned reference layer comprises asynthetic anti-ferromagnetic element.
 5. The magnetic cell of claim 1wherein the second ferromagnetic pinned reference layer comprises asynthetic anti-ferromagnetic element.
 6. The magnetic cell of claim 1wherein the ferromagnetic free layer switches between a high resistancedata state and a low resistance data state due to spin torque transferinduced by a current passing through the magnetic cell.
 7. The magneticcell of claim 1 wherein the first ferromagnetic pinned reference layerhas a first blocking temperature and the second ferromagnetic pinnedreference layer has a second blocking temperature that is less than thefirst blocking temperature.
 8. The magnetic cell of claim 1 wherein thefirst ferromagnetic pinned reference layer comprises a syntheticanti-ferromagnetic element having a first blocking temperature and thesecond ferromagnetic pinned reference layer comprises a syntheticanti-ferromagnetic element having a second blocking temperature that isless then the first blocking temperature.
 9. The magnetic cell of claim1 wherein the free magnetization orientation direction is orthogonal tothe second reference magnetization orientation direction.
 10. A spintorque transfer magnetic cell comprising: a ferromagnetic free layerhaving an in-plane free magnetization orientation direction thatswitches between a high resistance data state and a low resistance datastate due to spin torque transfer induced by a current passing throughthe magnetic cell; a first ferromagnetic pinned reference layer having afirst reference magnetization orientation direction that is parallel oranti-parallel to the free magnetization orientation direction; a firstoxide barrier layer between the ferromagnetic free layer and the firstferromagnetic pinned reference layer; a second ferromagnetic pinnedreference layer comprising a permanent magnet and having an in-planesecond reference magnetization orientation direction that is orthogonalto the free magnetization orientation direction; and a second oxidebarrier layer between the ferromagnetic free layer and the secondferromagnetic pinned reference layer.
 11. The spin torque transfermagnetic cell of claim 10 wherein the first ferromagnetic pinnedreference layer comprises a synthetic anti-ferromagnetic element. 12.The spin torque transfer magnetic cell of claim 10 wherein the secondferromagnetic pinned reference layer comprises a syntheticanti-ferromagnetic element.
 13. The spin torque transfer magnetic cellof claim 10 wherein the first ferromagnetic pinned reference layer has afirst blocking temperature and the second ferromagnetic pinned referencelayer has a second blocking temperature that is less than the firstblocking temperature.
 14. The spin torque transfer magnetic cell ofclaim 10 wherein the first ferromagnetic pinned reference layercomprises a synthetic anti-ferromagnetic element having a first blockingtemperature and the second ferromagnetic pinned reference layercomprises a synthetic anti-ferromagnetic element having a secondblocking temperature that is less then the first blocking temperature.15. The spin torque transfer magnetic cell of claim 10 wherein the freemagnetization orientation direction is orthogonal to the secondreference magnetization orientation direction.
 16. A spin torquetransfer magnetic cell comprising: a ferromagnetic free layer having anin-plane free magnetization orientation direction that switches betweena high resistance data state and a low resistance data state due to spintorque transfer induced by a current passing through the magnetic cell;a first ferromagnetic pinned reference layer having a first referencemagnetization orientation direction that is parallel or anti-parallel tothe free magnetization orientation direction; a first oxide barrierlayer between the ferromagnetic free layer and the first ferromagneticpinned reference layer; a second ferromagnetic pinned reference layercomprising a permanent magnet and having an in-plane second referencemagnetization orientation direction that is orthogonal to the firstreference magnetization orientation direction; and a second oxidebarrier layer between the ferromagnetic free layer and the secondferromagnetic pinned reference layer.
 17. The spin torque transfermagnetic cell of claim 16 wherein the first ferromagnetic pinnedreference layer comprises a synthetic anti-ferromagnetic element. 18.The spin torque transfer magnetic cell of claim 16 wherein the secondferromagnetic pinned reference layer comprises a syntheticanti-ferromagnetic element.
 19. The spin torque transfer magnetic cellof claim 16 wherein the first ferromagnetic pinned reference layer has afirst blocking temperature and the second ferromagnetic pinned referencelayer has a second blocking temperature that is less than the firstblocking temperature.
 20. The spin torque transfer magnetic cell ofclaim 16 wherein the first ferromagnetic pinned reference layercomprises a synthetic anti-ferromagnetic element having a first blockingtemperature and the second ferromagnetic pinned reference layercomprises a synthetic anti-ferromagnetic element having a secondblocking temperature that is less then the first blocking temperature.